In a turbomachine, the incoming air splits into two streams, a primary stream that flows in the more central region, and a secondary stream that surrounds the primary stream circumferentially. The radially-outer boundary of the primary stream, i.e. the region where the primary and secondary streams meet, forms substantially a cylinder of axis parallel to the main axis of the turbomachine. On coming into contact with the nose of the turbomachine, the central region of the primary stream flows while turning and following the wall of the nose of the turbomachine, which nose is conical in shape, flaring in the flow direction of the air as far as the inlet to a circumferential passage, where the wall of the nose becomes progressively parallel to the main axis of the turbomachine. A portion of the conical wall of the nose, upstream from the passage in the flow direction of the primary stream, is constituted by a platform referred to as the fan platform and that serves to carry blades. These blades serve to compress the air of the primary streams and to impart axial rotary motion thereto. By progressing along the cone between the blades of the fan up to the inlet to the passage, while also turning, the primary stream is also compressed radially between the wall of the cone and the secondary air stream, and its inner boundary moves away from the main axis of the turbomachine. This mechanism serves to obtain air at the inlet to the passage that is compressed and that presents energy that has been increased since it is turning relative to the main axis of the turbomachine at a mean radius (average distance from the main axis) that is greater than its initial mean radius.
The primary and secondary streams separate at the inlet to the circumferential passage surrounding the nose of the turbomachine, the primary stream penetrating into the passage while the secondary stream flows along the radially-outer surface of the annular wall defining the outside of the passage. Within the passage there are situated blades that extend radially and that enable the primary stream to be compressed further. It is desirable for the air of the primary stream penetrating into the passage to be as highly compressed as possible, so as to facilitate the compression work performed on said air by the low pressure compressor of the passage. The efficiency with which the air of the primary stream is compressed by the fan blades increases with increasing number of blades. Nevertheless, such blades are expensive. It can be advantageous to reduce the number thereof. In addition, reducing the number of blades reduces the weight of the platform, and thus its inertia. Nevertheless, in a fan platform having fewer blades, it is more difficult to compress the air that passes between the blades. This makes the work to be performed by the compressor downstream therefrom more difficult to achieve.
The present invention seeks to remedy those drawbacks, or at least to attenuate them.